This project is to define the function of RNA editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) in hematopoietic and leukemia stem cells. Particularly the requirement of RNA editing activity of ADAR1 in these stem cells will be determined. ADAR1 is an essential protein for embryonic and adult hematopoiesis, while the leukemia cells are more susceptible to the gene deletion that codes ADAR1. Whereas hematopoietic repopulation in the recipients by transplanted hematopoietic stem cell (HSC) is suppressed in the absence of ADAR1, the homing and self-renewal capacities are intact. Massive cell death only occurred in the differentiating progenitor cells. Moreover, it have been found in a mouse leukemia model removal of ADAR1 causes massive leukemia cell death including the leukemia stem/progenitor cells within one week in all the treated animals. Delineation of the discrepant impacts of ADAR1 on normal and leukemia stem cells (LSC) might shed a light on the characterization of the leukemia stem cells and lead to a discovery of a molecular target for leukemia treatment. The goal of this project is to delineate the different impacts of ADAR1 on normal and LSC and develop a therapeutic strategy to eliminate leukemia stem cells by targeting ADAR1. Although ADAR1 is identified as an RNA editing enzyme and RNA editing has been shown to play critical roles in stem cells and other biological processes, multiple attempts to date have been unable to identify an editing target that accounts for the death of normal hematopoietic and leukemia cells of ADAR1 knockouts. It is challenging to determine RNA-editing is required for ADAR1's effects on leukemia and LSCs in the absence of a known causal edited transcript, particularly because editing-independent functions fo ADAR1 have recently been described. It is hypothesized here that the RNA editing activity of ADAR1 does not account for its function in leukemia cells. This proposal is to seek a definitive answer whether the editing activity of ADAR1 is necessary for the proliferation and differentiation of LSCs. Therefore this project will first generate a Knock-In (KI) mouse model in which an inactive ADAR1 for RNA editing is expressed from mutated endogenous ADAR1 gene. Then the impact of inactive ADAR1 on the hematopoietic and LSCs will be analyzed in the ADAR1 KI animals. HSC and LSC as well as the mature cells will be observed and compared to wild type and ADAR1 knockout cells. The knowledge obtained from this project will help to understand the LSC and leukemia development and eventually contribute to improve the leukemia treatment.